Mandy Raab said she was told by vets that her beloved pet Maddy needed to be down because of a deteriorating leg condition.
But Miss Raab, from Telford, Shropshire, and partner Paul Saxton said they later discovered the wrong cat - Tammy - had been put down.
The couple had arranged to have the rescue cats, Tammy and Maddy, neutered last week.
But vets said that Maddy was suffering from a deteriorating leg condition and needed to be put to sleep.
Miss Raab said: ''Maddy's leg had started to get very bad - she couldn't jump up or climb on anything, and she got to the point where she couldn't even stand up without falling over again.
'When my partner walked in to the vets and saw Maddy standing there, he asked the nurse straight away where Tammy was. Her face dropped and she said 'are you sure that's Maddy?'
'What pet owner wouldn't recognise their own cat though? The vet came out and was very apologetic - he said he was very sorry, but there had been a mistake, and they had put the wrong cat to sleep.
'I just howled in pain - I was distraught, no-one could comfort me. Tammy was a perfectly healthy 10-month-old kitten. There was no need for her to die.'
Source: Daily Mail